Recently, a liquid crystal display device has been widely used as a display device of an information device such as a notebook-type personal computer, a word processor and the like, or as a display device of a video device such as a portable television, a video movie, a car navigation system and the like, by taking advantage of a characteristic in which the liquid crystal device is light and thin, and consumes small electricity. Such liquid crystal display device typically has a structure in which a liquid crystal panel is illuminated from behind by a built-in lighting unit for obtaining a bright display screen. The liquid crystal panel comprises at least a liquid crystal cell comprising a pair of transparent substrates such as glass substrates, with a liquid crystal layer interposed therebetween, and a pair of polarizers provided so as to retain the liquid crystal cell between them. As a structure of such a lighting unit, there is an edge light type in which a light guiding plate is disposed on a rear surface of the liquid crystal panel, and a linear light source such as a fluorescent discharge tube is disposed on an end face of the light guiding plate, for example. The edge light type excels in achieving a thin liquid crystal display device and a light emitting surface thereof with a uniform luminance. So, this is commonly adopted as a backlight type of the liquid crystal display device used in the notebook-type personal computer or the like. In the liquid crystal display device used in the portable television, the car navigation system and the like, the edge light type using two or more fluorescent discharge tubes, or the edge light type using an L-shaped or a U-shaped fluorescent discharge tube is commonly adopted, for obtaining a thin and luminous liquid crystal display.
In the edge light type in which the fluorescent discharge tube is disposed on the end face of the light guiding plate, it is important to reduce the size of the liquid crystal display device by storing components such as the light guiding plate, the fluorescent discharge tube, a lead wire or the like within a casing, for improving a characteristic of the liquid crystal display device such as portability and compactness.
Today, the liquid crystal display device is used in all household appliances in addition to the above-described uses. For example, the liquid crystal display device is commonly used in products used in severe surroundings, such as outside, owing to a portability thereof. Therefore, it becomes more important to consider about a warranty for a function of such products.
FIG. 4 is a cross-sectional view schematically showing a structure of the lighting unit of a conventional edge light type, comprising the fluorescent discharge tube, and of the liquid crystal display device comprising the lighting unit. FIGS. 5(a) and 5(b) are views schematically showing a structure of the lighting unit in FIG. 4, wherein FIG. 5(a) is a plan view and FIG. 5(b) is a side view. As shown in FIGS. 4, 5(a), and 5(b), the lighting unit comprises a flat-plate shaped transparent light guiding plate 1, configured to transmit light, an L-shaped fluorescent discharge tube 2 provided close to two of four sides of the light guiding plate 1, a reflecting sheet 3 for inhibiting light emitted from the fluorescent discharge tube 2 from emanating out of the light guiding plate 1, a casing 9 for holding these components, and a light correction sheet 4 provided on a light emanating surface side of the light guiding plate 1,
The reflecting sheet 3 is bent so as to enclose the fluorescent discharge tube 2 disposed on an end face E1 of the light guiding plate 1, and an end portion of the reflecting sheet 3 is bonded to the light emanating surface of the light guiding plate 1 by a double face adhesive tape 10. Thereby, the light emitted from the fluorescent discharge tube 2 is guided into the light guiding plate 1 from the end face E1 of the light guiding plate 1 without leakage. The reflecting sheet 3 is also disposed on a rear surface (an opposite surface of the light emanating surface) of the light guiding plate 1, for reflecting the light emanating from the rear surface of the light guiding plate 1, and for returning the light into the light guiding plate 1. Furthermore, the reflecting sheet 3 is bent along an end face E2 of the light guiding plate 1 so as to cover the end face E2, on which the fluorescent discharge tube 2 is not provided. Thus structured, the light emanating from the end face E2 of the light guiding plate 1 is reflected by the reflecting sheet 3 and is returned into the light guiding plate 1. By thus returning the light emanating from the light guiding plate 1 into the same by the reflecting sheet 3, the light (i.e., an illumination light) emanating from the light emanating surface of the light guiding plate 1 is increased, thereby enabling the light to be used more efficiently.
As the reflecting sheet 3, a white resinous film having a high reflectivity is used, for example. The reflecting sheet 3 may be structured by using separate sheets disposed so as to correspond to a periphery of the fluorescent discharge tube 2, the rear surface of the light guiding plate 1, and the end face E2 of the light guiding plate 1. In this case, an integral-type sheet that continuously covers these portions is used. By using the integral-type reflecting sheet 3, a thin lighting unit, a cost reduction thereof, and a reduction of the number of assembly processes thereof are realized.
The light correction sheet 4 is disposed on the light emanating surface of the light guiding plate 1, for obtaining a uniform and highly luminous illumination light. The light correction sheet 4 comprises a plurality of optical sheets of various specifications as necessary. Herein, two light correction sheets 4 are provided, as shown in FIG. 4.
The light guiding plate 1 and the fluorescent discharge tube 2, which are covered with the reflecting sheet 3, and the light correction sheet 4, are held by the casing 9. The lighting unit is formed by these components thus stored in the casing 9. The casing 9 is frame-shaped so as to have an opening portion J and comprises an edge portion 9a and a side portion 9b. By forming the opening portion J, a display region A of a liquid crystal panel 11 to be described below is located within the opening portion J, and the edge portion 9a of the casing 9 encloses the display region A, in a plan view. A through-hole G which communicates with outside is provided on the side portion 9b of the casing 9, for a weight saving and an installation by a user. Furthermore, the light correction sheet 4 and the light guiding plate 1 are disposed so as to form a space (hereinafter, referred to as a clearance F) between them and an inner wall of the casing 9. By forming the clearance F, it becomes possible to absorb a dimension difference by the clearance F, when the light correction sheet 4, the light guiding plate 1, and the casing 9 expand with different expansion rates due to a temperature variation, thereby resulting in the dimension difference. Therefore, it becomes possible to inhibit occurrence of distortion or the like due to the dimension difference.
Furthermore, on the edge portion 9a of the casing 9 of the lighting unit, a liquid crystal panel 11 comprising a display rear surface side polarizer 13a, a display front surface side polarizer 13b, and a liquid crystal cell 111 comprising at least a pair of transparent substrates which retain a liquid crystal layer between them is provided. The liquid crystal panel 11 is fixed to the edge portion 9a of the casing 9 by a double face adhesive tape 14. And, a front cover 12 is provided so as to cover these components. A liquid crystal display device is obtained by thus disposing the liquid crystal panel 11 on the light emanating surface side of the lighting unit.
Since the display rear surface side polarizer 13a is disposed on the casing 9, as described above, a space 15 is formed between a bottom surface of the polarizer 13a and the light emanating surface of the lighting unit (an upper surface of the light correction sheet 4, to be more precise). Therefore, there is a problem that, the liquid crystal cell 111 deflects as much as the space 15 when a user accidentally pushes the display surface of the liquid crystal display device, so that the liquid crystal cell 111 comprising the glass substrates or the like is broken.
On the other hand, as described above, since the casing 9 has the through-hole G, dust might enter the casing (i. e., the lighting unit) from outside through the through-hole G. The entering dust might also enter the display region such as a vicinity of the light correction sheet 4 and the light guiding plate 1 through the clearance F. In the lighting unit and the liquid crystal display device thus structured, an outer size thereof is limited for obtaining a compact device, and thus it is often impossible to provide a component for inhibiting entry of the dust. Therefore, the dust entering from outside as described above, or being generated inside when assembling the device passes through the clearance F and enters a space between the light guiding plate 1 and the light correction sheet 4, or between the light correction sheet 4 or the like and the liquid crystal panel 11. The dust entering the space interrupts an output light when displaying an image, thereby causing non-uniform luminance to occur, or conceals pixels, thereby causing display defects. Also, there is a possibility that a component of the display region side such as the light correction sheet 4 is damaged by a friction between components and the dust. Once the dust enters a space between the liquid crystal panel 11 and the lighting unit, it is very difficult to clear the dust away without disassembling the device. Therefore, it is very important to inhibit the dust from entering the display region.